Alkali metal polysilicates have long been known as protective coatings that modify the permeability or surface characteristics of polymeric films and other articles. Lithium (Li), sodium (Na), and potassium (K) polysilicates are known to be effective as protective coatings for a variety of surfaces. For example, Japanese Patent Publication No. H7-18202, published Jan. 20, 1995 refers to a water-insoluble, coating material and binder for use on metals, synthetic resins, glass, wood, cement, etc, which contains mixtures of an aqueous solution of sodium or potassium silicate and an aqueous solution of lithium silicate, where the ratio of the sodium or potassium silicate to lithium silicate is in the range of 1-3.
As another example, Hecht and Iler, Canadian Patent No. 993,738, describe a gas and liquid-impermeable coating for polymeric substrates comprising lithium polysilicate having a mole ratio of SiO.sub.2 to Li.sub.2 O of about 1.6 to 4.6. However, polymeric articles are known to become hazy after certain polysilicate coatings have been incorporated. The tendency of sodium (Na) polysilicate coatings to effloresce, i.e., to become covered by powdery crystalline material as a result of atmospheric exposure, has been documented [Weldes and Lange, Ind. Eng. Chem., 61(4):28-44 (1969)]. This characteristic has similarly been observed for lithium polysilicate coatings by the present inventors. In contrast, pure potassium polysilicate coatings do not effloresce, but suffer severe loss of barrier performance above 50% relative humidity. Pure lithium polysilicate coatings, on the other hand, exhibit little or no loss of barrier performance over the same relative humidity range.
There remains a need in the art of barrier coatings for coating compositions and methods which overcome the above deficiencies, and are widely available as vapor, gas and/or aroma barriers for polymeric articles, e.g., packaging products.